1. Field of the Invention
This invention concerns a method and a system for improved in situ bioremediation of groundwater by increasing the operational longevity of an in situ microbial filter emplaced in an aquifer. The longevity of the biofilter is increased by selecting a stable bacterial isolate and enhancing its longevity with one or more additives. In particular, this invention provides a method for producing a microbial filter of sufficient catalytic density and thickness, and for increasing the bacterial replenishment interval, via improved attachment and detachment characteristics of the cells and their endogenous catalytic stability under the devised in situ attachment conditions.
Attachment and detachment characteristics of the bacteria of the invention are enhanced by screening an otherwise pure homogeneous population of a single bacterial strain for stable isolates, preferably rosette cluster forming isolates, and improving these properties by modifications of the injection cell buffer.
The bioremediation system of the invention comprises a bioreactor for growing a biofilter of bacterial cells, a biofilter, a means for emplacement of the biofilter in situ in a contaminated water aquifer and a means for extracting remediated water through the biofilter.
2. Background and Related Art
Groundwater aquifers at many industrial, research and defense-related sites are often contaminated with chlorinated volatile organic compounds (VOCs). While "pump and treat" is currently the standard method for remediating these compounds, in situ bioremediation using bacterial filters described, for example, in Geomicrobiol., 8:133-146 (1991), continues to generate much interest.
Methanotrophic bacteria, containing the soluble form of methane monooxygenase (sMMO), are known to oxidatively degrade a number of chlorinated aliphatic hydrocarbons, including trichloroethylene (TCE) (Appl. Environ. Microbiol., 57:228-235 and 1031-1037 (1991)). A specific in situ microbial filter approach described in Hydro Sci. J., (IAHS), 38:323-342 (1993) utilizes resting-state (nondividing) cells of the methanotroph, Methylosinus trichosporium OB3b. In this filter strategy, cells that have been previously grown in a bioreactor are injected into the subsurface ahead of a dilute, migrating contaminant plume. Ideally, as the contaminated groundwater flows into this zone, the attached microbial population degrades the contaminant(s) at a rate that keeps pace with the rate of transport, and the groundwater then exits clean. However, an in situ pilot field test of a M. trichosporium OB3b filter, used for a narrow, shallow, fast moving TCE plume at a contaminated site revealed that the microbial filter had only short term longevity (several days), in terms of an efficient removal of TCE, and therefore, very limited utility in groundwater remediation (Environ. Sc. Technol., 30: 1982-1989 (1996). Need for frequent replacement of the filter biomass makes this treatment method uneconomical and impractical.
To be economically viable and operationally achievable in the field, the in situ microbial filter will have to degrade TCE and several other chlorinated ethanes over a period of several weeks before it is replenished. Several parameters influence the filter's ability to achieve this: the finite biotransformation capacity of the attached resting cells for contaminants, the attachment/detachment properties of the injected bacteria with respect to the natural geological sediments or an introduced in situ sand trench and the long term endogenous stability of the whole-cell sMMO activity. Without improved and enhanced functional or operational longevity, the whole-cell rate of hydrocarbon contaminant catalysis and the biotransformation capacity would decay too rapidly to extremely low values over time. This, in turn, would prove to be impractical for in situ bioremediation applications.
Therefore, it would be desirable to provide a laboratory-based predictive methodology for creating an economical bacterial filter at a field site, a biofilter having an improved and enhanced longevity that would allow a continuous remediation without a need to replace the microbial filter more than once in about 6-8 weeks.
Because bacteria are particles, the colloid filtration theory provides a basis for studies of cell transport through saturated, subsurface media. Advection, dispersion, deposition, and entrainment are all processes that can influence this movement (Geomicrobiol., 8:133-146 (1991)). Yet overall, it is generally accepted that cell surface hydrophobicity and cell/solid electrostatic interactions are the most important factors which influence bacterial deposition onto, and retention by, aquifer sediments. Experimentally, physical properties (e.g., grain size) as well as chemical properties (ionic strength and pH) have been shown to influence the attachment density of bacteria to aquifer sands and mineral surfaces. Generally, an ionic strength of .about.0.01 molal is needed to promote maximal attachment, but the process is not electrolyte specific. Additionally, chemical alterations of both the aqueous and solid media and of the bacterial cell surface can be quite important. For example, it has been reported that Mg and Fe oxide coatings of aquifer sands increase bacterial attachment (J. Contam. Hydrol. 6:321-336 (1990).
While the need for improved microbial filter remediation of contaminated groundwater persists and while attempts at providing a means for such improved remediation have been made, due to a short lifespan of available microbial filters, such remediation efforts have not been successful.
It would be, therefore, advantageous to provide validatable methods and simulated conditions that would permit an increased longevity of any specific bacteria that might be used for an in situ microbial filter, especially as it relates to enhancing their attachment and detachment properties.
It is therefore a primary objective of this invention to utilize the resting-state M. trichosporium OB3b cells or other bacteria having improved attachment and detachment properties as an in situ biofilter for the treatment of ground water containing a chlorinated hydrocarbon contaminant, such as a TCE-contaminated plume.
All patents, patent applications and publications cited herein are incorporated by reference.